A rooster mates with the hen, depositing sperm in her oviduct. He sperm fertilizes one of the ovum or egg cells given off by the hen’s overy. The hen’s body forms an egg around the fertilized ovum. Once the shell of the egg has hardened it is laid in a nest.
If this fertilized egg is heated to the body temperature of the hen for about 21 days, a chick will hatch out of it.
The fertilized ovum is attached to the yoke of the egg. As the egg is incubated a light can be shine through the egg the progress. This is called candling the egg.